Final answer:
The design of the Plan of San Carlo alle Quattro Fontane by Francesco Borromini is partly oval and partly a cross, featuring complex convex-concave rhythms and deviating from traditional architectural forms of the time.
Step-by-step explanation:
The design of the Plan of San Carlo alle Quattro Fontane in Rome, Italy, is partly oval and partly a cross. The architectural work, commissioned by Cardinal Francesco Barberini for the Holy Order of the Trinity, began in 1638 and was consecrated in 1646 by architect Francesco Borromini. Borromini's design deviated from traditional forms, utilizing complex geometric shapes and multi-layered symbolism. His use of the oval intersects with Baroque sensibilities, distinguishing the church with its complex convex-concave wall rhythms. The description and historical analysis provided by Frank Dabell, Dr. Beth Harris, and Dr. Steven Zucker, elucidates the inventive nature of Borromini's architectural contributions during the Baroque period.